The short answer is because it would scare the pants off of us. But if
you must know the real scientific answer, well, nobody knows
actually:

The short answer is, researchers don't know exactly, although there
are several hypotheses as to why insects and other arthropods don't
get bigger, said insect physiologist Jon Harrison, at Arizona State
University in Tempe.

The first hypothesis is that insects' exoskeletons may not be strong
enough to allow them to get much bigger — that they'd have to
become impossibly thick. Harrison learned this theory as an established
fact during his training, but little experimental evidence to support
the idea exists, he said. The only study to look at this question found
that larger arthropods don't have thicker exoskeletons, he said. "So
there's no direct evidence for this," he said.